Multiple-effect evaporation systems are, of course, known for the concentration of solutions including black liquor as produced in the production of paper pulp. By and large, however, the operation of such systems is not economical. Costs are high because forced circulation evaporators are normally employed in; the known process and apparatus system.
Furthermore, incrustation of the pathways of the plant is common. This means that maintenance costs are high and shut down of the plant may be required to allow incrustation to be removed.